As horrible as it feels Alyss, I can guarantee you that it is about him, and his problems, and not about you. Most men just cannot talk about it, and women tend to internalize things they do not understand. He could have E.D. or erectile dysfunction, and unless they are self assured and open emotionally (very few are open, especially about this topic) they will never bring it up, and they will not even admit it.
I know that doesn't help you much, but it is the truth. I hope it helps you some. This is so sad. I see it all the time. Like what mom said, it is this way for many, even the otherwise, happily married. I have been there and done that, and until my SO came clean of all the secrets, we didn't have a chance.
Life is so complicated, try not to internalize things that you don't understand. Let it be enough to admit that you don't understand, and let it go. If you have faith in God, ask Him to explain it to you, but, if He doesn't, it's okay. You can always ask why, and then, leave it with Him...even if you don't know the reason now, I assure you that it isn't about you...it is about him.
If the King of the Universe desires you, and He does...how could you possibly be undesirable?
truth
Just wanted to share this exerpt from a poem entitled, 'Desiderata':
Be yourself.
Especially, do not feign affection.
Neither be cynical about love;
for in the face of all aridity and disenchantment
it is as perennial as the grass.
Take kindly the counsel of the years,
gracefully surrendering the things of youth.
Nurture strength of spirit to shield you in sudden misfortune.
But do not distress yourself with dark imaginings.
Many fears are born of fatigue and loneliness.
Beyond a wholesome discipline,
be gentle with yourself.
You are a child of the universe,
no less than the trees and the stars;
you have a right to be here.
And whether or not it is clear to you,
no doubt the universe is unfolding as it should.
Therefore be at peace with God,
whatever you conceive Him to be,
and whatever your labors and aspirations,
in the noisy confusion of life keep peace with your soul.
With all its sham, drudgery, and broken dreams,
it is still a beautiful world.
Be cheerful.
Strive to be happy.
Max Ehrmann, Desiderata, Copyright 1952.